SHENZHEN SUNYUAN TECHNOLOGY CO., LTD.
FXXXXD/S-1W SERIES DC-DC CONVERTER
CONNECTING DC-DC CONVERTERS IN SERIES
Galvanic isolation of the output allows multiple converters to be connected in series simply by connecting the positive output of one converter to the
negative of another (see figure 1). In this way, non-standard voltage rails can be generated, however, the current output of the highest output voltage
converter should not be exceeded. When converters are connected in series, additional filtering is strongly recommended as the converters switching
circuits are not synchronized. As well as a summation of the ripple voltages, the output could also produce relatively large beat frequencies. A
capacitor across the output will help, as will a series inductor.
CONNECTING DC-DC CONVERTERS IN PARALLEL
If the available power output from a single converter is inadequate for the application, then multiple converters can be paralleled to produce a higher
output power. However, it is not recommended to parallel converters to generate a higher power output, please select our higher power output
modules or contact us for a customized solution.
It should be noted that it is always preferable to parallel multiple converters of the same type. For instance, if a 2.5W converter is required, then
either 2pcs AxxxxS-2W should be used or 3 AxxxxS-1W, not an AxxxxS-2W and an AxxxxS-1W. The reason for this is that the output voltages are
not sufficiently well matched to guarantee that an AxxxxS-2W would supply twice as much as an AxxxxS-1W and the situation would occur where
there was only 1W being drawn from the AxxxxS-2W and 1.5W from AxxxxS-1W. Even with paralleled converters of the same type, loading will be
uneven, however, there is only likely to be around a 10% difference in output load when the output voltages are well matched.
When connecting converter outputs, it should be remembered that the switching will not be synchronous, hence some form of coupling should be
employed. One possible solution is to use a diode feed, this is suitable mainly for 12V and 15V output types only where the diode voltage drop
(typically 0.6V) will not significantly affect the circuit functionality (see figure 2). With 5V and 9V supplies the diode drop is generally too large to
consider as a suitable means of connecting paralleled converters. This method also has a beat frequency that will superimpose itself over the ripple
of the two converters, this can be reduced by using an external capacitor at the paralleled output. The preferred method of connecting converters in
parallel is via series inductors on the output (see figure 3). This configuration not only has a lower loss of voltage than the diode method, but by
suitable choice of inductor and an additional external capacitor, the beat frequency can be significantly reduced, as will the ripple from each
converter. Suitable values are given in the table below (see table 1), these typically reduce the beat and ripple frequencies by a factor of 10.
One of the main features of the majority of Sunyuan components DC-DC converters is the high galvanic isolation capability. This allows several
variations on circuit topography by using a single DC-DC converter. The basic input to output isolation can be used to provide either a simple
isolated output power source, or to generate different voltage rails and /or dual polarity rails. These configurations are most often found in data
processing, instrumentation and other noise sensitive circuits where it is necessary to isolate the load and noise presented to the local power supply
rails from that of an entire system. Usually local supply noise appears as common mode noise at the converter and does not pollute the main system
power supply rails. The isolated positive output can be connected to the input ground rail to generate a negative supply rail if required. Since the
output is isolated from the input the choice of reference for the output side can be relatively arbitrary, for example an additional single rail can be
generated above the main supply rail or offset by some other DC value.
Regulated converters need more consideration than the unregulated type for mixing the reference level. Essentially the single supply rail has a
regulator in its positive output rail only, hence referring the isolated ground will only work if all the current return is through the DC-DC and not via
other external components, such as diode bias, resistor feed. Have an alternative return path can upset the regulation and the performance of the
system may not equal that of the converter. In one word, the isolation features are to generate two completely independent power supply but also
united perfectly through the modern technology. This has been widely used in the application of mixed circuits, A/D, D/A and other circuits that are
sensitive to noise.
All Sunyuan isolated DC-DC Converters have a fixed characteristic frequency at which the device operates. This fixed frequency allows filtering
that is relatively simple compared to pulse-skipping types. In a pulse-skipping converter a large range of frequencies are encountered as the device
adjusts the pulse interval for loading conditions.
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Printed in P.R.C January 2007 Page 3 of 4